Vacuum circuit interrupters



March 14, 1961 J. M. LAFFERTY 2,975,255

VACUUM CIRCUIT INTERRUPTERS Filed July 24, 1958 @IX/ll/l in van tor.-

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//s Attorney United States Patent VACUUM CIRCUIT INTERRUPTERS James M.Lafferty, Schenectady, N.Y., assignor to General Electric Company, acorporation of New York Filed July 24, 1958, Sel. No. 750,614

6 Claims. (Cl. 200-144) The present invention relates generally tovacuum type electric circuit interrupters uniquely adapted for use ininductive circuits wherein the current interrupted is of low magnitude.

The phenomena of electric current interruption in atmosphere or higherpressures, on one hand and in high vacuum on the other hand areextremely complex and basically different in nature. So also, are theproblems which must be overcome in producing operative devices toperform either function. Thus, for example, one greatproblem innon-vacuum circuit interrupters is the deterioration of the contactsthereof due to the formation of oxides of the contact metal and otherchemical com pounds due to the presence of atmospheric air, a protectivegaseous atmosphere or an oil bath. Any compound so formed is readilyseparated from the interrupter contacts. Such a continuous formation andremoval of non-metallic compounds rapidly causes the interruptercontacts to erode and deteriorate.

In the vacuum interrupter, on the other hand, the complete absence ofgases or liquids in contact with the interrupter contacts virtuallyprecludes the formation of contact-eroding oxides and compounds.

Other problems, however, not generally considered or significant in thenon-vacuum circuit interrupter are of prime importance in theconstruction of vacuum circuit interrupters. Vacuum interruptersdesigned to interrupt alternating currents operate substantially asfollows:

When an arc is struck between the electrodes of a vacuum interrupter, asfor example, by the opening of a pair of switch contacts maintained in avacuum of 10" mm. of mercury pressure or less, the arc vaporizes some ofthe electrode material. If the contacts have been properly processed toremove therefrom all occluded and sorbed gases, only ionized metallicvapor is present in the arc. The are continues to exist until thealternaaing current sought to be interrupted falls to a natural zerovalue, as normally occurs once each alternation. When this occurs, thearc is distinguished, and the ionized metallic particles rapidly diffuseto the cold vacuum chamber walls where they are cooled and de-ionized.When the interrupted current seeks to rise again, the are usually cannotbe reestablished because of the high dielectric strength of the vacuumseparating the interrupter contacts. Thus, in an alternating currentvacuum circuit interrupter, the current is usually completelyextinguished at the first naturally occurring instantaneous zero valueof current.

The foregoing occurs satisfactorily in vacuum circuit interruptersoperating currents of high current ratings, namely of 500 amperes orgreater.v For lower values of alternating current, the phenomenongenerally de nominated as chopping occurs. .At these low alternatingcurrent values, the arc, rather than persisting until the first naturalcurrent zero, is abruptlyv extinguished at some low current value andfalls instantaneouslyfrom that value to zero. The value of current atwhich instantaneous. arc extinction occurs is denominated the.

chopping current of the device. Typical chopping currents of prior artvacuum interrupters may be from 10 to 40 amperes. While chopping invacuum interrupters associated with capacitive circuits may betolerated, it is unacceptable with inductive loads because of voltagesurges induced therein by the high rate of change of current time(di/dt) when an interrupter chops. This may be seen from therelationship:

L n e where V=the surge voltage induced by chopping 1 =the choppingcurrent.

L=the equivalent inductance of the circuit C=the equivalent capacitanceof the circuit,

and surge impedances of inductive devices commonly have values of tensof thouands ohms.

For most industrial inductive circuit loads, it is necessary to reducethe chopping current level of a vacuum circuit interrupter to a value ofbelow 4 amperes and, in most instances to a value of below 2 amperes.

Accordingly, it is an object of the present invention to provide vacuumcircuit interrupters suitable for inter- 4 rupting low value alternatingcurrents in inductive circuits.

tion thereof to a value of several amperes.

Still another object of the invention is to provide vacuum circuitinterrupters having arc electrodes which minimize chopping and havehardness and brittleness characteristics which avoid permanent weldingtogether of the contacts or shattering thereof.

In accord with the present invention a vacuum circuit interrupter isprovided in the form of an evacuable chamber capable of maintaining avacuum of less than 10* mm. of mercury, which chamber contains a pair ofopposed arc-electrodes adapted to be the terminal points of an electricare carrying the current to be interrupted. The portions of the arcelectrodes which serve as such terminal points are substantially free ofoccluded and sorbed gases and are formed primarily of an alloy ofantimony, bismuth or mixtures thereof, the remainder being copper,silver or mixtures thereof. While the electrodes may be constructedsubstantially entirely of the above class of alloys, these alloys may,in one embodiment, be utilized as an impregnating agent to fill thealternating current illustrating the effect of the chopping phenomenon.

In Fig. 1, an interrupter chamber 10 comprises a wall member 11 whichmay be cylindrical in shape and is constructed of a suitable insulatingmaterial, having at the ends thereof a pair of metallic end members 12and 13 closing the volume therein to form an interrupter chamber.Suitable seals 14 are provided between casing 11 and end members 12 and13 to render the interrupter chamber vacuum tight.

A further object of the invention is to provide a vacuum circuitinterrupter capable of reducing the value ofinstantaneous alternatingcurrent changes upon interrup-' Located within chamber are a pair ofseparable contacts or are electrodes and 16 shown in their closedcircuit or engaged position. Upper contact 15 is a stationary contactsuitably attached electrically and mechanically to a conducting rod 17which, at its upper end, is united electrically and mechanically withend member 12. Lower contact 18, mounted upon, and electrically unitedwith a suitable conducting rod 18, is movable and is connected throughbellows or an equivalent vacuum tight member premitting reciprocatingmotion. Terminal mounting rod 18 projects through a suitable orifice inend member 13, and suitable actuating means may be connected thereto tocause a reciprocating motion of rod 18 to cause contact 16 to enter intoengagement with, and be removed out of engagement with, contact 15. Forcertain types of interrupters, as for example, vacuum fuses and lightingarresters, electrode 16 need not be movable but may be spaced apart fromelectrode 15 a suitable distance. The electrical circuit which is soughtto be interrupted by the interrupter device may be completed by makingsuitable connections to contact terminal 21, electrically andmechanically mounted upon end member 12, and terminal 22, electricallyand mechanically mounted upon rod 18. A suitable insulator shield, suchas metallic cylindrical member 23, capped with an arc-preventing ferrule24, is interposed between electrodes 15-16 and insulator 11 to preventthe latter from becoming coated with metal and becoming electricallyshort-circuited.

The volume within interrupter chamber 10 is suitably evacuated throughan exhaust tubulation (not shown) during the final assembly thereof. Forproper operation of the interrupter as a vacuum-type interrupter ofalternating currents, the pressure within chamber 10 must be maintainedat a pressure at least 10 mm. of mercury, but is preferably maintainedwithin the range 10- to 10'' mm. of mercury. The foregoing requirementis essential for the operation of the devices as vacuum interrupters ofalternating currents. This requirement is necessary because, in orderthat the current-carrying arc struck between electrodes 15 and 16 beextinguished at the first occurring current zero value, there must besubstantially no ionizable gas present within chamber 10. The occurrenceof ionization may be substantially prevented if the possible breakdownpaths between electrodes 15 and 16, or their respective supports, aresmall with respect to the mean free path of an electron within theatmosphere obtained within the device. This mean free path is designatedas the statistical distance which an electron may travel withoutcolliding with agas molecule at a given pressure. These conditions maybe established within the devices of the present invention only when thepressure within interrupter 10 chamber is below 10- mm. of mercury andpreferably below 10- mm. of

mercury.

In Fig. 2 of the drawing there is shown, in graphical form, anillustration of the choppingphenomena. In Fig. 2 the instantaneous valueof a sinusoidal alternating current, sought to be interrupted by avacuum circuit interrupter, is plotted for one-half cycle. As thecurrent depicted by curve A rises from instantaneous value of zero, thecontacts, as for example, contacts 15 and 16 in Fig. 1, are separated,causing the establishment of an arc discharge therebetween. This arcdischarge is sustained exclusively by the metal evaporated from contacts15 and 16 by the heat generated at the contact surfaces by the are. Theterminal points of the are are known respectively as cathode and anodespots. As a matter of practice, most of this evaporation occurs at thecathode, or negatively maintained electrode. It is, however, difficultto predetermine which electrode is negative ata particular instant whenan alternating current circuit is interrupted. In Fig. 2,; the value ofcurrent flowing in the arc follows its natural course along the sinusoidof curve A and, for high current arcs (those above 500 amperes),

follows the dotted line until a zero value is reached. At this instant,the arc is extinguished and the energized metallic ions between theelectrodes rapidly diffuse to the cold walls of members 12, 13 and 23where they are cooled and deionized. The are remains extinguishedbecause, when a high voltage builds up between contacts 15 and 16, thearc is not reestablished, due to the high dielectric strength of thevacuum separatingthe contacts.

In the operation of vauum circuit interrupters wherein the currentsought to be interrupted is of a relatively low value (below 500amperes) the instantaneous current value does not follow the dotted linecontinuously to a zero value but, rather, at some low current value,denominated by I and occurring at time 0, the arc is abruptly andprematurely extinguished. This results in an instantaneous change ofcurrent from a value of 1,, to zero. The value I is referred to hereinas the chopping current value for a particular device. As will bereadily appreciated, this almost instantaneous change of current from Ito zero results in a high rate of change of current with time (di/dt)and may result in the production of extremely high surge voltages ininductive loads which may be connected thereto. These surges may causethe breakdown of insulation and are generally highly injurious toelectrical equipment.

In the copending application of T. H. Lee and I. D. Cobine, S.N. 750,784filed concurrently herewith, it is disclosed that chopping of low levelalternating currents in vacuum circuit interrupters may be caused by anunstable condition resulting from a preponderance of are: constrictingmagnetic pressure over opposing vapor pressure in the region of one ofthe electrodes between which the arc is struck. As a remedy to thechopping problem, it is shown therein that vacuum circuit interruptersmay be constructed utilizing arc-electrodes comprising particular highvapor pressure materials so that sufiicient vapor pressure is availableto counterbalance the arc-constricting magnetic pressure.

Although the contact materials contemplated by the aforementioned Leeand Cobine application are quite satisfactory in providing low values ofchopping current in the interruption of low current alternatingcurrents, certain of these materials in elemental form tend to berelatively soft or to have low melting points. As the result of thesecharacteristics, certain of the pure metals may be readily eroded andmelted and, when melted by a relatively high temperature are, tend toweld and stick. Additionally, certain low melting point electrodes havean undue tendency to weld under the heating effect of momentary currentswhich flow before the arc-electrodes are separated and an arc is struck.

In accord with present inventionat least one of electrodes 15 and 16and, in some instances both of these electrodes, are composed of alloys,one constituent of which is selected from the group consisting ofbismuth, antimony and mixtures thereof while the major con stituent isselected from the group consisting of copper, silver and mixturesthereof. The advantages gained by the alloys of the aforementionedconstituents are the attainment of higher melting points, low thermalconductivity, and an unexpected but not excessive degree of hardness andbrittleness, highly desirable in vacuum circuit interrupters. As usedherein, the melting point of an alloy composition is used to identifythat temperature atwhich a body of the alloy composition becomesentirely molten.

Insofar as these characteristics are concerned, the characteristics maybe obtained in satisfactory fashion if copper-bismuth alloys areprepared in the range of 10 s 10 to 15% by weight antimony, theremainder being silver. If silver-bismuth is the alloy system chosen,the alloy may be from to 20% by weight of bismuth, the remainder beingsilver.

If, in all of the alloy systems utilized, the percentage of the highvapor pressure component (bismuth or antimony) is below 10% or lower,the contribution ofthis constituent to supply a quantity of metallicvapor to prevent extinction of the arc, and consequent chopping becauseof vapor starvation, is insufiicient. If, on the other hand, thequantity of the high vapor pressure material exceeds the maximum statedvalues, the advantages of hardness and higher melting points are notobtained.

As an illustration of the advantages in the attainment of high meltingpoints by utilizing the specific alloys set forth herein over utilizingelemental high-vapor pressure metals it is to be noted that whilebismuth has a melting point of 271 an alloy of 10% bismuth and 90%copper exhibits a melting point of approximately 980 C. and an alloy of35% bismuth and 80% copper has a melting point of 860 C. Likewise, analloy of 10% bismuth and 90% silver exhibits a melting point ofapproximately 840 C. and an alloy of 20% bismuth and 80% silver has amelting point of 825 0., both as compared with the melting point of 271C. for pure bismuth. Additionally, whereas antimony in its elementalform has a melting point of 630 C., an alloy of 10% antimony and 90%copper has a melting point of approximately 935 C. while an alloy of 20%antomony and 80% copper has a melting point of-870 C. Likewise, an alloyof 10% antimony and 90% silver has a melting point of approximately 835C. and an alloy of antimony and 85% silver has a melting point of 8000., all of which are compared with a melting point of 630 C. forelemental antimony.

That the melting point of vacuum alternating current are interrupters isimportant in preventing excessive melting of the contacts and, moreimportant, in preventing the contacts from welding together, may readilybe seen from the fact that the cathode spot of a vacuum arc interrupter,such as is utilized to interrupt currents of several hundred amperesmagnitude at 600 volts, often has a temperature of from 2500 K. to 3500K.

A further unexpected advantage obtained in vacuum arc interruptersutilized for the interruption of alternating current in accord with thepresent invention, by uitlizing the disclosed alloys of silver or copperwith antimony or bismuth, is the highly desirable intermediatebrittleness of the alloys formed by the ranges disclosed herein. As iswell known in the art, welding together of the contacts of a vacuumswitch is a serious problem. If a weld between two contacts composed ofrelatively ductile elements such as bismuth does tend to form, the weldmay withstand the initial shock of an attempt to separate the contacts,so that a permanent juncture therebetween is formed. The alloys of thisapplication, however, are all quite brittle and, should an initialwelding action tend to take place, this brittleness, with its attendantlack of tensile strength, allows for such a weld to be broken so thatthe contacts do not permanently remain joined together The brittlenessof the alloys of the present invention does not approach the point, asdoes that of some elemental high-vapor pressure materials, whereshattering of the contact becomes a problem As a condition precedent tosatisfactory operation of any vacuum switch it is necessary that theelectrodes between which an arc is to exist momentarily be substantiallyfree of occluded and sorbed gases This requires that the electrodes bepreviously conditioned in order to remove from them all sorbed andoccluded gases. For this reason, among others, prior art vacuum circuitinterrupters have not heretofore been constructed utilizing any of thehigh vapor pressure constituents of the alloys disclosed and claimedherein. Rendering a material completely free of occluded and sorbedgases is most effectively done by outgasing and baking the material at 6extremely high temperatures. Thus, operative vacuum circuit interruptershave in the past utilized refractory meta-ls as the electrodes thereof,since these materials may be heated to temperatures in the range of2000' C. without appreciably evaporating or melting. Obviously, suchtreatment cannot be utilized upon the contact of the alloy independentlyas for example, by repeated. arc meltings in a vacuum. The constituentsthereof are then once again melted in vacuum, mixed and cast in the formdesired as an electrode or contact.

It is, of course, necessary to provide a standard for occluded andsorbed gases for vacuum interrupter contacts in order that theinterrupters be operative.

the devices of the present invention, contacts, when placed in avacuumized test chamber a few liters in volume and subsequently deeplyeroded by a repetitive arcing, as for example, with a voltage ofcommercial power at a current of 100 amperes or more, the pressure levelin the container, a few cycles after arcing, does not rise substantiallyfrom its initial value in the absence of get-ters and pumps even if theinitial pressure is the order of 10* mm. of mercury. Analytically, thisrequirement may be stated in the relationship that the contact mate-"rial must contain less than 10- atomic parts of all gases. Sincerepeated arc erosion in accord with such tests consumes a substantialportion of the contact material, it provides a satisfactory test for thedetermination whether the arc electrodes are substantially free of alloccluded and sorbed gases since even the infinitesimal presence of suchwould cause a substantial rise in the presence of the test chamber. As aspecific example of arc-electrode material utilized in accord with thepresent invention, contacts of an alloy of 20 weight percent bismuth,copper, approximately /2" in diameter and substantially free of alloccluded and sorbed gases successfully interrupted currents of 1500amperes at 15000 volts repeat-' edly. In low-current tests of the samecontact material the chopping level was held to below 1 ampere and thecontacts did not weld or stick.

After the arc-electrodes of the interrupter of Fig. 1

have been prepared in accord with the foregoing procedures, the entireassembly is assembled, connected to.

a suitable vacuum pump and evacuated. While evacuated to a pressure ofat least lO- mm. of mercury, or less, the entire device is suitablyraised to a temperature of at least 500 C. in order to bake out all ofthe components thereof to free them from surface adsorbed gases so thatan operative pressure may be maintained therein even under hightemperature conditions caused by repeated arcing between the electrodesthereof. Suitably the device may be evacuated for approximately 10 hoursat a pressure l0 mm. of mercury to satisfactory perform this function.

Although it is an object of the present invention to provide harder,intermediately brittle and higher melting point, high vapor pressurecontact electrodes for vacuum circuit interrupters, and the alloys ofthe present invention serve this object, the mechanical characteristicsof the alloys alone need not be relied upon for resistance to wear inthe fabrication of vacuum circuit interrupters in accord with thepresent invention. Thus, electrodes 15 and 16 of the device of Fig. 1,in addition to being composed entirely or substantially entirely of thedisclosed and claimed alloys, may comprise a refractory However, it hasbeen found that, once the- Accordingly, the elec-- Such a standard isprovided by a requirement that, for use in:

matrix or, for example a porous mass of tungsten or molybdenum; intowhich the high-vapor pressure, highmelting point alloysdisclosedhereinbefore may be added by'infiltration' by melting in intimatecontact in high vacuum. In such an arrangement, the mechanical strengthof the porous refractory matrix adds greatly to, the utility of theinterrupter and greatly prolongs its life.

As is mentioned hereinbefore, presently available evidence indicatesthat chopping, the premature extinction of an alternating current arebefore a normally occurring current zero, isprimarily a phenomena whichoccurs at the cathode electrode. Accordingly, when suitable precautionsare taken so that a given electrode may be madetheinstantaneous cathodewhen an alternating current is interrupted, in order that the advantagesof the present invention be obtained, it is only necessary that oneofthe alloys disclosed herein for use as electrodes for vacuum circuitinterrupter-s, be utilized for the cathode electrode. Accordingly, it isclearly within the scope of the present invention that operative vacuumcircuit interrupters which exhibit low values of chopping currents,without undesirable welding or sticking disadvantages, may beconstructed wherein only one of the arc electrodes is fabricated of thealloys disclosed and claimed herein. It is, however, contemplated in apreferred embodiment of the invention that both electrodes beconstructed of these materials since, in general,-it is difficult' topredetermine which electrode will be the instantaneous cathode at theinstant the circuit is inter rupted and it may not be feasible toprovide means for establishing such certainty.

While the invention has been disclosed herein" with respect to certainembodiments thereof, it is apparent that many modifications and changeswill readily occur to'those skilled in the art. Accordingly, by theappended claims I intend to cover such modifications and changes asfall'within true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuated to a'pressure lower than 10- mm. ofmercury; a pair of electrical terminals adapted for connection in analternating current circuit; a pair of electrodes located within saidenvelope, connected in circuit between said terminals and disposed inspaced-apart relationship during circuit interrupting operation to allowfor the establishment of a. circuit interrupting arc therebetween; eachof said electrodeshaving'a-region on which the respective electrodesspots for a low current are may be established, one of said electroderegions com-prising an alloy selected from the group consisting of 10 to35% by weight of bismuth the remainder being copper, 10 to by weight ofantimony the remainder being copper, 10 to 15% by 2. The circuitinterrupter of claim 1 wherein the electrode region is an alloy ofapproximately 20% by weight of bismuth, the remainder being copper.

3. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuated to a pres"- sure lower than 10* mm. ofmercury; a pair: of electrical terminals adapted for connection in analternating current circuit; a pair of electrodes located within saidenvelope, connected in circuit between said terminals anddisposed inspaced apart relationship during circuit interrupting operation to allowfor the establishment of a circuit interrupting arc therebetween; one ofsaid'elec trodes comprising an alloy selected from the group consistingof 10 to 35% by weight of bismuth the remainder being copper, 10 to 20%by weight of antimony the remainder being copper, 10 to 15% by weightantimony the remainder being silver, and 10 to 20% by weight of bismuththe remainder being silver; said electrodes being substantially free ofall occluded and sorbed gases.

4. The circuit interrupter ofclaim 3 wherein the electrode alloyconsists essentially of approximately 20% by weight of bismuth theremainder being copper.

5. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuatedto a pressure lower than 10' mm. of mercury;a pair of electrical terminals, adapted for connection in an'alternatingcurrent circuit; a pair of electrodes located within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship, during circuit interrupting operation to allow for theestablishmentof a circuit interrupting are therebetween; one of saidelectrodes comprising a porous refractory body the interstices of whichare substantially filled with an impreg- 20% by weight of bismuth theremainder being copper. 7

References Cited in the file of this patent UNITED STATES PATENTSMillikan et al. Dec. 9, 1930 Ruben May 14, 1940

